The present invention relates to a method and apparatus for achieving a desired moisture content uniformly throughout a web of sheet material, particularly a paper web which has been dried so as to be within the hygroscopic range. As is well known, during the manufacture of sheet material such as paper, this sheet material is invariably dried so as to be within the hygroscopic range. The vapor pressure of the water which is bound in the web is lower than that of free water at the same temperature. The lower the moisture content of the web, the greater the difference between the vapor pressure of the water bound therein and the vapor pressure of free water at the same temperature. Thus, the partial pressure of the water in the paper at a given temperature is lowered as the web becomes drier.
It is particularly to be noted that the vapor pressure of hygroscopic water is the same as the pressure of air at a temperature in excess of 100.degree. C., the latter temperature being higher as the web becomes drier. It is thus to be understood that in principle the web is also in equilibrium with water vapor under atmospheric pressure and superheated to the extent of a relatively small number of degrees Centrigrade. The above physical circumstances constitute the starting point of the present invention.
As is well known, during the manufacture of sheet material such as paper, the latter sheet material is directed over a plurality of cylinders during drying of the sheet material. One of the recognized problems in connection with achieving a paper of high quality is the difficulty of obtaining uniform ultimate moisture content in the web, particularly over the entire width of the web. This difficulty arises from the fact that a multiple-cylinder drying apparatus cannot be constructed and operated in such a way that the drying conditions will be uniform over the entire width of the web which is dried, so as to be capable of achieving a constant specific drying over the entire working breadth of the web. Even in the case where the moisture content and base weight of the web which arrives at the drier from the presses are uniform, the moisture profile of the web in the drying section will lack uniformity for a number of reasons as set forth below.
In the first place, the non-uniformity of the drying atmosphere will result in a lack of uniform drying of the web. This lack of uniformity in the drying atmosphere results from the unavoidable presence of spaces, resembling compartments or pockets, which are defined by the cylinders, rolls, the drying felt or wire, and by the web itself, which is to be dried, with these spaces having a poor ventilation. In the absence of separate ventilation means, air flow in these confined spaces is induced only by the moving surfaces, under the effect of pumping caused by natural water and possibly by the wires which are permeable to air. The air is usually humid in the central region of the paper machine, and this latter circumstance tends to reduce the specific drying at this central region. The greater the width of the paper machine, the greater are the above problems.
In the second place, the margins of the web of sheet material tend to dry to a greater extent than the sheet material between the margins thereof, because the parts of the drying cylinders which engage the margins of the web and extend outwardly beyond them increase the heating surface available for drying the marginal portions of the web.
In the third place, it is difficult to control the conditions inside the drying cylinders. For example, the presence of condensate draining members results in differences in the thickness of the condensate layer and accumulation of uncondensed gases at certain parts of the drying cylinders. The result is unequal heat transfer to the wall of each drying cylinder, and of course non-uniform drying is unavoidable.
The most common expedient utilized for reducing drying problems, in the present state of the art, is to provide the drying apparatus with various types of devices relating to air technology. In part these devices serve only the purpose of equalizing the state of the drying air by increasing the drying air flow in the direction transverse to the direction of travel of the web through the machine, by utilizing various air-blowing nozzles from which the air is blown into spaces transversely with respect to the direction of web travel, these nozzles in addition inducing secondary air flow from the spaces which are situated beside the machine.
It is also known in the art to utilize, for example, so-called doctor blowing devices, and, in machines which utilize drying wires, so-called pocket ventilating devices, by means of which it is attempted to achieve not only a uniform state for the drying air but, in contrast, differences in the state of the drying air in order to produce local differences of specific drying. Thus, the prior art devices may in this latter connection be considered as being utilized to produce intentional errors in the form of unequal local specific evaporation, but means of which actual errors resulting from other factors can be compensated. However, the effect of these prior art devices are limited, particularly inasmuch as truly significant differences of drying conditions in the state of the drying air are difficult to achieve and in addition the effect is smaller as the area of the web where the correction is applied becomes narrower. This latter factor results because turbulence prevents obtaining major differences in the state of the air at relatively narrow zones of the web.
As is known, it has been attempted to achieve equalization of the moisture profile of the web by utilizing devices in the interior of the drying cylinders. Such devices include, for example, profile-correction cylinders which are heated by electrical resistances having a heating power which is arranged so as to be adjustable in blocks which are distributed transversely with respect to the direction of web travel. In fact, it has even been suggested to utilize ceramic insulation which is placed close to the ends of the cylinder and inside the cylinder so as to attempt to reduce the detrimental excessive drying of the web at the margins thereof.
It is also in the art to condition the paper web by passing the latter, after it has been dried, through a treating machine therein the web remains in an accurately controlled atmosphere for a time long enough to achieve throughout the web at least an approximate equilibrium condition with respect to the atmosphere in which the web is located, so that in this way the ultimate moisture content of the web will have a uniformity which is greater as the interval during which the web remains in a controlled atmosphere becomes greater. However, when utilizing high-speed paper machines, such treatment becomes impractical because the treating machine must be made undesirably large and involves an undesirably high cost. The equilibrium referred to above is the state in which the vapor pressure of the water in the web is the same as that of the air which contacts the web.